In a Radio Frequency (RF) frontend, the output of the power amplifier (PA) and the input of the Low Noise amplifier (LNA) are known to be connected to a matching network. The purpose of the PA matching network is to provide the output of the PA with the optimum load impedance for maximum efficiency and a certain desired output power level. The purpose of the LNA matching network is to provide the input of the LNA with the optimum source impedance for the lowest possible noise figure. The outputs of these matching networks are connected to a switching device, which will connect the attached antenna either to the PA matching network (transmit mode) or the LNA matching network (receive mode). RF frontends with PA and LNA having a separate output and input respectively are also known.
Most of the time an antenna is designed for a situation in which the direct environment of the antenna does not change. However, in practice, the antenna will experience a varying environment. This change in the environment can be initiated e.g. by placing the antenna on a human body or on a surface with other properties than what was designed for. This change in environment has an impact on the electrical input impedance and other performance parameters of the antenna.
The change in the electrical input impedance can be measured in several ways. Some of these techniques are outlined below.
“A precise and sensitive X-band reflecto-meter providing automatic full-band display of reflection coefficient”, F. C. de Ronde, IEEE Transactions on Microwave Theory and Techniques, pp. 435-440, July 1965 discloses a method of voltage peak detection at two points along a transmission line. This provides information on the voltage standing wave ratio (VSWR), which can be used to estimate impedance.
“Antenna impedance mismatch measurement and correction for adaptive CDMA transceivers”, D. Qiao, D. Choi, Y. Zhao, D. Kelly, T. Hung, D. Kimball, M. Li, P. Asbeck, IEEE International Microwave Symposium Digest, pp. 783-786, June 2005 discloses voltage peak detection at three nodes. This provides information on the real and imaginary part of the impedance, but in a non-orthogonal manner.
“Frequency agile antenna tuning and matching”, J. R. Moritz, Y. Sun, IEE 8th International Conference on HF Radio Systems and Techniques, no 474, pp. 169-174, July 2000 discloses a mixer based quadrature detector in conjunction with a VSWR bridge. This provides information on the reflection coefficient, but not on impedance without calculations.
The disadvantage of the first three methods is the need for additional sensing elements in the RF path that are lossy, introducing extra parasitic capacitance and requiring additional space leading to off chip implementations. Furthermore, these methods only work for the PA and can only work if the PA is activated during the detection process. This consumes a substantial amount of current during the detection process. Most of the currently available impedance detectors involve off-chip components, which add to the bill-of-material.